Microfluidic sensors have been fabricated in order to optically analyze small samples of fluids. For example, sensors have been made comprising a cuvette having optically neutral walls in which a small sample of a fluid to be analyzed is placed, and then a light beam is directed through the cuvette so that properties of the fluid such as the optical refraction, absorption and transmission characteristics can be detected by analysis of the transmitted and emitted light. Since the cuvettes are relatively small in order to place a small fluid sample in position for passage therethrough of a light beam, the path length of the light beam through the fluid in the cuvette is short. Moreover, the diameter of the light beam is generally small, so that the interaction cross-section between the fluid and the light beam also is accordingly small. Hence, in order to obtain a sufficiently strong signal for analysis, such systems typically cause the light beam to make a series of passes through the cuvette before the beam is directed into instrumentation for detecting refraction, absorption and transmission. Such systems are necessarily complex, and may therefore be both cumbersome and delicate. Further microfluidic sensors have been fabricated which comprise planar microfluidic channels incorporated into a microchip. However, the inefficiencies of multiple required passes of an analytical light beam through such microfluidic channels still remain.